While development has been advanced on many kinds of vacuum members such as vacuum parts represented by a vacuum vessel or pipes with which a vacuum system is constructed, a demand for an ultra high vacuum at a higher level, in recent years, has been increasingly enhanced in the field of this kind along with increase in new demands for a charged particle accelerator, a thin film forming apparatus, a surface analyzer and others. Furthermore, in a superconducting accelerating cavity employing niobium as a structural material thereof, it has been raised a demand for an accelerating cavity showing a high Q-value under ultra high vacuum in a high accelerating electric field, that is a so-called high performance accelerating cavity, and it has also been desired to reduce a construction cost of an accelerator and a running cost thereof since, in such a situation, the accelerator has a tendency to require higher energy in operation and in turn, a larger scale in itself, leading to requirement for a number of vacuum members including an accelerating cavity and others.
In order to realize a super-high vacuum state of vacuum members, required is at least a high vacuum degree in the range from about 133.322×10−7 to about 133.322×10−9 Pa (10−7 to 10−9 torr) or not more than the range. Actually, however, a gasifiable component adsorbed on and occluded as a solid solution in an inner surface of a vacuum member is outdiffused and separated from a surface and an inner surface thereof when evacuation is started and gradually released into a vacuum system, which lowers an ultimate vacuum degree. On the other hand, in a case of an accelerating cavity, hydrogen occluded as a solid state raises a surface resistance of the inside of the member, having resulted in a problem that sufficient performance in accelerating a particle cannot be obtained. Gasifiable components adsorbed and occluded as a solid solution usually include nitrogen, carbon monoxide, water and the like in addition to hydrogen, while hydrogen is a main component of about 90% in content of a total of the gases. Especially, in a superconducting accelerating cavity, hydrogen is occluded as a solid solution in the vicinity of a surface layer, very close thereto, to thereby form niobium hydride during cooling and increase a surface resistance and to invoke reduction in acceleration performance. Accordingly, it has been required to develop a technique to reduce hydrogen and water adsorbed on and occluded as a solid solution in an inner surface of a vacuum member to the lowest possible level. Note that the term “an inner surface of a vacuum member” means a surface and a bulk region in the vicinity of a surface layer of a vacuum member.
A material of a vacuum member is subjected to various kinds of forming techniques such as cutting, bending, press working, bulging, electron beam welding and others. A strain, a damage, a surface wrinkle, embedding of foreign matter or the like generated in the forming steps cause various kinds of surface defective layer including a work-affected layer and others on or in an inner surface of a vacuum member, leading to not only an adverse influence on a vacuum degree but also increase in surface resistance in application to high frequency. Therefore, it has been common that such a surface defective layer is applied with mechanical polishing, electrochemical polishing (hereinafter, also referred to as electrolytic polishing) or chemical polishing to thereby render an inner surface of a vacuum member smooth and clean.
Especially in a case where a vacuum member is a superconducting accelerating cavity, insufficient smoothness and cleanliness of an inner surface of the vacuum member exerts, naturally, an adverse influence on a vacuum degree and causes an increase in surface resistance of the member, thereby disabling a stable accelerating electric field and a high Q-value to be acquired. Therefore, after a mechanical polishing is applied onto an inner surface of the cavity, electrolytic polishing or chemical polishing is effected thereon to thereby achieve a smooth and clean surface.
For example, described in Patent Literature 1 is that a centrifugal barrel polishing, which is one of mechanical polishing methods, is applied on an inner surface of a niobium superconducting accelerating cavity and, then, electrolytic polishing or chemical polishing is effected thereon to thereby render an inner surface of the cavity smooth and clean.
In recent years, however, it has been found that with one of the polishing techniques applied, hydrogen is occluded as a solid solution in an inner surface of a superconducting accelerating cavity, and hydrogen thus occluded as a solid solution causes an increase in surface resistance value of the cavity to thereby lower a acceleration performance or the like.
Chemical polishing usually uses concentrated phosphoric acid, concentrated nitric acid, hydrofluoric acid and the like as a polishing solution, and has an advantage that the polishing can be effected only by immersing a work piece in a solution with a high polishing speed. On the other hand, there have been recognized a problem of reduction in Q-value in a high accelerating electric field of a superconducting accelerating cavity obtained by chemical polishing in an early period and a problem of occlusion of hydrogen as a solid solution.
In electrolytic polishing, the following polishing solution are generally employed: a mixture of concentrated sulfuric acid and hydrofluoric acid, a mixture of hydrofluoric acid and butanol, and the like and a superconducting accelerating cavity obtained by electrolytic polishing has a great advantage to have no reduction in Q-value even in a high accelerating electric field. Therefore, especially in a case where a superconducting accelerating cavity is manufactured, it has generally understood that use of electrolytic polishing is advantageous over use of chemical polishing and therefore, has been increasingly adopted, in recent years, in many of institutes where a study on an accelerator is conducted. Examples of such an electrolytic polishing include electrolytic polishing described, for example, in Patent Literature 2, and the like.
Electrolytic polishing is slower in a polishing speed than chemical polishing (chemical polishing: electrolytic polishing=10 to 20:1) and requires complicated jigs. Problems have been arisen that hydrogen is occluded as a solid solution in proportion to an electrolytic polishing time, which exerts an adverse influence on characteristics of an accelerator and that vacuum annealing is required for dehydrogenation after the electrolytic polishing.
As described above, the following complicated steps have been required, for example, in a conventional manufacturing process for a superconducting accelerating cavity as a vacuum member: (i) various kinds of forming, (ii) mechanical polishing, (iii) chemical polishing or/and electrolytic polishing, (iv) vacuum annealing, (v) light electrolytic polishing or light chemical polishing after the vacuum annealing, and the like. Accordingly, in the present invention, no process is adopted in which hydrogen temporarily occluded as a solid solution in a member is removed by a different means such as vacuum annealing and, instead, occlusion as a solid solution of hydrogen is prevented, before actually being occluded as a solid solution, in a process in which the member is formed and polished, and a technique of this kind has not been known at all thus far to the public. With a surface-treating process according to the present invention adopted, occlusion as a solid solution of hydrogen in a forming step and a surface treating step of polishing is blocked or extremely alleviated; therefore, the following steps are not necessary altogether: (iv) vacuum annealing and (v) electrolytic polishing or a chemical polishing conducted in succession to the vacuum annealing. Accordingly, since a manufacturing process for a vacuum member, especially a superconducting accelerating cavity, can be made greatly simpler, the present invention can be said an industrially extremely useful technique capable of reducing a manufacturing cost. Furthermore, the present inventors have discovered that by applying electrolytic polishing after mechanical polishing, hydrogen is occluded as a solid solution in a vacuum member even during electrolytic polishing. There has been no knowledge thus far of a technique realizing prevention of occlusion of hydrogen as a solid solution into a vacuum member during mechanical polishing or electrolytic polishing. With a surface-treating process according to the present invention adopted, it is possible to suppress occlusion of hydrogen as a solid solution into a vacuum member not only during mechanical polishing but also during electrolytic polishing following the mechanical polishing; therefore, a superconducting accelerating cavity can be made of a high performance and in addition thereto, vacuum annealing after the polishing can be unnecessary.
Patent Literature 1:
Japanese Unexamined Patent Publication No. 2000-71164
Patent Literature 2:
Japanese Patent No. 2947270.